Method of reducing pattern collapse in high aspect ratio nanostructures

ABSTRACT

A method is provided for treating the surface of high aspect ratio nanostructures to help protect the delicate nanostructures during some of the rigorous processing involved in fabrication of semiconductor devices. A wafer containing high aspect ratio nanostructures is treated to make the surfaces of the nanostructures more hydrophobic. The treatment may include the application of a primer that chemically alters the surfaces of the nanostructures preventing them from getting damaged during subsequent wet clean processes. The wafer may then be further processed, for example a wet cleaning process followed by a drying process. The increased hydrophobicity of the nanostructures helps to reduce or prevent collapse of the nanostructures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to semiconductor devices. More specifically, thepresent invention relates to the production of semiconductor deviceswhere wet chemical treatments are used on a wafer with high aspect rationanostructures that are sensitive to pattern collapse during processing.

2. Description of the Related Art

Semiconductor devices are fabricated using a long complex procedure. Oneportion of the procedure involves etching features into a stack ofmaterials on a silicon wafer. The stack of materials may comprise asingle layer of silicon based material such as SiO or SiN, or the stackmay comprise multiple layers of materials such as SiO, SiN, TEOS,polysilicon or silicon in different orders within the stack. The stackmay be formed by a number of methods, including physical vapordeposition, chemical vapor deposition, electrochemical deposition andmolecular beam epitaxy, for example. Once the stack of materials iscreated, a photoresist layer is applied. This photoresist layer is usedas a mask for etching. Many methods of etching may be used includingmethods of wet etching and dry etching. After the etching, thephotoresist layer is usually removed, often by a plasma ashingprocedure.

During the fabrication, the wafers are subjected to wet processing suchas wet cleaning. Wet cleaning is helpful to prepare the surfaces and toremove residue left behind by some of the other processing. The cleaningprocess usually consists of chemical treatment in combination withmegasonics, jets and/or other particle removal techniques followed byrinsing and drying. The drying may include bulk liquid removal from thesurface by spin off, vacuum suction, Marangoni effect with isopropylalcohol or combination of these commonly known techniques

A wafer may go through multiple occurrences of these steps during theentire fabrication process. Hence, as device features shrink on a waferand as much liquid is used in the processing, strong capillary forcesmay exert enough force to collapse the structures during drying steps.

SUMMARY OF THE INVENTION

To achieve the foregoing and in accordance with the purpose of thepresent invention, a method of processing a wafer used in fabricatingsemiconductor devices is provided. The method teaches processing thewafer in such a way as to reduce or eliminate collapse of high aspectratio features on the wafer. High aspect ratio features are formed in asilicon based layer that has been produced on the wafer. The sidewallsof the features are treated to make them more hydrophobic. A wetprocessing of the wafer is performed on the wafer and then the wafer isdried.

In another embodiment, a method of processing a wafer used infabricating semiconductor devices is provided. High aspect ratiofeatures are formed in a silicon based layer on the wafer. A wetprocessing of the wafer is performed. The wet processing includes wetcleaning the wafer, depositing a primer on the wafer that modifiessurface properties of the features so as to increase the hydophobicityof the surfaces of the features and rinsing the wafer. After the wetprocessing, the wafer is then dried.

These and other features of the present invention will be described inmore details below in the detailed description of the invention and inconjunction with the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIGS. 1A and 1B show high level flow charts of some embodiments of theinvention.

FIGS. 2A-2C show an exemplary wafer undergoing a damaging wet processingand drying.

FIGS. 3A-3G show an exemplary wafer undergoing select steps of anembodiment of the invention.

FIGS. 4A-4G show an exemplary wafer undergoing select steps of anembodiment of the invention.

FIGS. 5A-5H show an exemplary wafer undergoing select steps of anembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

Oxides and nitrides of silicon grown by low-temperature oxidation (LTO),chemical vapor deposition (CVD) and implantation have been traditionallyused in front-end-of-the-line (FEOL) processing for electrical andthermal isolation, masking and encapsulation in high aspect rationanostructures. The use of such materials in FEOL applications hascontinued to offer the selectivity to reduce feature sizes and increaseaspect ratios to achieve the desired densities of devices for 32 nmintegrated circuit (IC) fabrication and beyond. Aspect ratios are nowcommonly in the range of 10:1 to 25:1 and possibly higher. However, asthe critical dimensions continue to shrink and aspect ratios continue toincrease in FEOL applications, problems associated with processing suchdensely fabricated nanostructures have surfaced and are anticipated topose tremendous challenges for wet clean processes. One commonlyobserved problem has been the collapse of densely packed high aspectratio nanostructures used for shallow trench isolation.

FIG. 2A shows a wafer 201 together with a set of nanostructures 202.Feature collapse occurs during wet cleaning and subsequent drying. FIG.2B shows the nanostructures 202 of FIG. 2A during a wet cleaning wherethe fluid 204 has gathered between the nanostructures 202. Fluid entersthe features during wet processing and strong capillary forces may exertsufficient force to damage the delicate nanostructures. In addition, thesurface tension forces of the drying liquid tend to pull surfaces ofadjacent nanostructures into contact. These forces are often exacerbatedby the lack of balance of pressures across the many features due tonon-uniformity of the fluid amounts gathered in the features, as well asnon-uniformity in drying. These forces often lead to the collapse of thefeatures individually or also through bridging. Bridging occurs when atleast two adjacent nanostructures collapse against one another andbecome adhered together. The sidewalls of adjacent nanostructuresthemselves may adhere together, or a residue material may gather betweenthe nanostructures, joining them together. FIG. 2C shows two examples offeature collapse. One example is the single collapse of a nanostructure206. The other example shows a bridging of two nanostructures 205. Othertypes of feature collapse are possible as well. Feature collapse can bea significant problem in semiconductor fabrication and can lead todefects in the circuits produced.

Some methods of reducing feature collapse have been explored. Examplesinclude rinsing with ultra-low surface tension liquids such as2-propanaol γ=22 dynes/cm, use of fluorinated organic surfactants (HFE,γ=14 dynes/cm) supercritical carbon dioxide drying, and drying usingsimilar approaches at elevated temperatures. Some of these techniqueshave met with limited success; however, such techniques are costly andoften require elaborate setups. For example, supercritical carbondioxide requires high pressures to go to the critical point.

A wafer may go through multiple occurrences of wet processing and dryingduring the entire fabrication process. As device features shrink on awafer, strong capillary forces may exert enough force to collapse thestructures during drying steps. A cost-effective and simple method ofprocessing the wafers is needed that reduces the occurrence of thecollapsing of high aspect ratio nanostructures during sequential wetprocessing. In light of this notion, the present invention outlines amethodology that can be used during single wafer wet cleaning of highaspect ratio nanostructures on a wafer to avoid collapse and sticking.

FIG. 1A is a high level overview of an embodiment of the invention. Atthe start of the method, at least one silicon based layer is depositedupon a wafer (step 102). A photoresist patterned mask is formed over thesilicon based layer (step 104) and features are etched into the siliconbased layer using the photoresist as a mask (step 108). The photoresistmask is then stripped away (step 112). A procedure is then used to makethe sidewalls of the features more hydrophobic (step 116). In oneembodiment, this procedure may include depositing a primer on the waferthat chemically alters the surface of the features as a first step inthe process flow without needing to wet the features. This step can becarried out by exposing the surface of interest to the vapor of themodifying agent prior to the wet processing. Wet processing is thenperformed on the wafer (step 120), which may include a sequential seriesof wet cleaning steps to clean the features of common residues leftbehind after etch. The wafer is then dried (step 124).

FIG. 1B is a high level overview of another embodiment of the invention.In this embodiment, the steps of making the sidewalls more hydrophobic(step 116) and the wet processing (step 120) are combined into one stepof making the sidewalls more hydrophobic during the wet processing (step124). For example this step can be carried out by exposing the surfaceof interest to a liquid solution that contains the modifying agent priorto or after the wet clean sequence. The liquid solution that containsthe surface modifying agent can be derived from solvents that aremiscible with the agent such as n-Hexane, Toluene, NPM, DMSO, Acetone,DMF, DMAC, or HFEs. The other steps may be left unchanged.

FIGS. 3A-3G show an example of a wafer with high aspect nanostructuresat selected steps of an embodiment of the disclosed method. FIG. 3Ashows the result after step 102 has been performed. A layer of siliconbased material 306 has been formed on the wafer 301. There are manyprocesses that may be used to form the silicon based layer 306. Forexample, the layer may be formed by physical vapor deposition, chemicalvapor deposition, electrochemical deposition or molecular beam epitaxy.While FIG. 3A shows a single uniform silicon based layer 306, it isimportant to note that multiple layers of materials may be useddepending on many factors, such as the intended use of the circuit beingfabricated or the specific fabrication process being used. Examples ofmultilayer structures include a common shallow trench isolation (STI)stack consisting of TEOS at the top followed by SiN, then PolySi, andthen Si at the bottom, or a stack consisting of SiN at the top followedby TEOS, then another layer of SiN, then PolySi, and then Si at thebottom. In another embodiment, a silicon based layer 306 is not formedon the wafer 301. Instead, the silicon wafer 301 is etched.

FIG. 3B shows the wafer 301 with the layer of silicon based material 306and a photoresist layer 305 that has been patterned. A photoresist layer305 is deposited on the silicon based material, often using a spincoating process and the photoresist is patterned using photolithography(step 104). The photoresist layer 305 is used as a mask to determinewhat silicon based material to remove and what to leave behind duringthe etching process. A wet or dry etching process (step 108) is used toremove the material not covered by the photoresist 305.

FIG. 3C shows the wafer 301 after the etching, showing thenanostructures 302 formed by the etching (step 108). At this point thephotoresist 305 is still present. After the etching, the photoresistmaterial may then be removed (step 112). The photoresist may be removedby a chemical stripping process or by an ashing process.

FIG. 3D shows a wafer 301 and four columns of the silicon based materialthat remains after etching and photoresist removal. The columns of thesilicon based material make up the nanostructures 302. It is importantto note that while the figures show a particular example of featuresthat may be etched on the wafer 301, other numbers and types of featuresare possible.

FIG. 3E shows the wafer 301 and nanostructures 302 after a layer ofprimer 303 has been deposited on the nanostructures 302 (step 116). Theprimer 303 may be a monolayer formed by self-assembly or any other knowndeposition process. Alternatively, the primer layer 303 may be thicker,such as a film. Some examples of possible surface modifying agentsinclude hexamethyldisiloxane (HMDS), and various alkoxysilanes andalkylsilanes. More specifically, fluorinated or long chain hydrocarbonbased trichlorosilane, dichlorosilane, monochlorosilane,trimethoxysilane, dimethoxysilane, methoxysilane, triethoxysilane,diethoxysilane, and ethoxysilane to name a few examples.

Adding the primer 303 to the nanostructures 302 helps reduce featurecollapse by modifying the surface properties of the nanostructures 302.The surface of the nanostructures 302 is chemically altered such thatthe stiction force between two adjacent surfaces is reduced orpreferably eliminated by making the surfaces of the nanostructures 302more hydrophobic. One example of a chemical modification that wouldmodulate stiction would be substituting polar hydroxyl groups of ananostructure surface (for example Si—O or Si—N, often used to fabricatehigh aspect ratio nanostructures) with nonpolar groups, such as Si—CH3,Si—R or Si—RF (where R is a hydrocarbon or fluoro substituted chain ofn-length). The Si—CH3 groups may be provided, for example, by HDMS(C₆H₁₈OSi₂). Another example of chemical modifier is 1H, 1H, 2H,2H-perfluorooctyltrichlorosilane (FOTS, C₈F₁₃H₄SiCl₃). The presence ofnonpolar groups provides a stable modified surface with DI water contactangles varying from 70-130° where development of excessive forces due toliquid meniscus formation can be prevented.

The primer 303 may be added before the wet phase of processing viareaction of the surface with its vapor. When added before the wet phase,the primer 303 additionally acts to minimize or prevent the excessivestictional forces that are often exerted by the adsorption of water onsurfaces that are in close proximity. A sample process flow using FOTSis first generating a stock solution of chemical modifier by mixing asolution of 0.1-50% FOTS by weight with anhydrous n-hexane, thenco-heating one drop of the stock solution with the sample to be modifiedin an oven at a temperature of 40-200° C. After approximately 2-300seconds, the stock solution evaporates completely and FOTS moleculereacts with the sample surface. A surface prepared this way has DI watercontact angle larger than 120°.

Alternatively, the primer may be added during the wet phase processing(step 126) by use of suitable solvents containing the modifying agent.This step could be applied either before or after the wet clean steps inthe sequential process flow. A sample process using FOTS is immersingthe sample into 0.01-50% by weight of FOTS in HFE-7100 (3M, Minneapolis,Minn.), Toluene, n-hexane, chloroform, or acetone for approximately 10seconds up to 1 hour under nitrogen, followed by rinsing ultrasonicallywith fresh HFE-7100, then drying with nitrogen.

FIG. 3F shows the wafer 301 undergoing a wet processing (step 120) suchas a wet cleaning after having the primer 303 applied. The liquid 304from the wet processing is gathered within the nanostructures 302 and isrepelled by the now more hydrophobic sidewalls of the nanostructures302. The increased hydrophobicity of the sidewalls of the nanostructures302 reduces the capillary forces present between the nanostructures 302and prevents the formation of a concave meniscus in the fluid gatheredwithin the features. FIG. 3G shows the wafer 301 after it has been driedfrom a DI water rinse (step 124). Optionally, the primer 303 may beremoved after the drying, for example by an oxygen or carbon dioxideflash process. This would return the wafer 301 and nanostructures 302 tothe state shown in FIG. 3D, but with any processing residue greatlyreduced or removed.

FIGS. 4A-4G show an example of a wafer 301 with high aspect rationanostructures 302 at selected steps of another embodiment of thedisclosed invention. FIG. 4A shows the result after step 102 has beenperformed. A layer of silicon based material 306 has been formed on thewafer 301. There are many processes that may be used to form the siliconbased layer 306. For example, the layer may be formed by physical vapordeposition, chemical vapor deposition, electrochemical deposition ormolecular beam epitaxy. While FIG. 4A shows a single uniform siliconbased layer 306, it is important to note that multiple layers ofmaterials may be used depending on many factors, such as the intendeduse of the circuit being fabricated or the specific fabrication processbeing used. Examples of multilayer structures include a stack consistingof TEOS at the top followed by SiN, then PolySi, and then Si at thebottom, or a stack consisting of SiN at the top followed by TEOS, thenanother layer of SiN, then PolySi, and then Si at the bottom.

FIG. 4B shows the wafer 301 with the layer of silicon based material 306and a photoresist layer 305 that has been patterned. A photoresist layer305 is deposited on the silicon based material, often using a spincoating process and the photoresist is patterned using photolithography(step 104). The photoresist layer 305 is used as a mask to determinewhat silicon based material to remove and what to leave behind duringthe etching process. A wet or dry etching process (step 108) may be usedto remove the material not covered by the photoresist 305.

FIG. 4C shows the wafer 301 after the etching, showing thenanostructures 302 formed by the etching (step 108). At this point thephotoresist 305 is still present. After the etching, the photoresistmaterial 305 may then be removed (step 112). The photoresist 305 may beremoved by a chemical stripping process or by an ashing process.

FIG. 4D shows a wafer 301 and four columns of the silicon based materialthat remain after etching and photoresist removal (step 112). Thecolumns of the silicon based material make up the nanostructures 302. Itis important to note that while the figures show a particular example offeatures that may be etched on the wafer 301, other numbers and types offeatures are possible.

In this embodiment, the sidewalls of the nanostructures 302 are mademore hydrophobic (step 116) by a roughening process. The surface of thenanostructures 302 can be reacted with a chemical substituent such thatthe surface morphology of the nanostructures 302 is changed. Forexample, while the photoresist 305 is spin-coated on the silicon basedlayer 306 the polymer resist can be exposed to a fluorine (F) and oxygen(O) mixture plasma to induce polymer re-deposition on the substrate. There-deposited polymer generated by the plasma reaction is not a smooththin film hence it can be used as a mask to etch the underlyingsubstrate. An alternating deposition and etch process can be used tovary the surface roughness and achieve the required topography toproduce a super-hydrophobic surface via a subsequent C₄F₈ plasma thinfilm coating process. The change results in rough interfaces 401, asshown in FIG. 4E, with increased surface area for subsequent reactionswith surface modifiers such that the surface becomes more hydrophobic.Roughening may be implemented by RIE texturing using polymerzing plasmaand dry or vapor phase fluoride etching at increased temperature, forexample.

FIG. 4F shows the wafer 301 undergoing a wet processing (step 120) suchas a wet cleaning after having surfaces of the nanostructures roughenedto increase surface area for subsequent surface modification reactions.The liquid 304 from the wet processing is gathered within thenanostructures 302 and is repelled by the now more hydrophobic sidewallsof the nanostructures 302. The increased hydrophobicity of the sidewallsof the nanostructures 302 reduces the capillary forces present betweenthe nanostructures 302 and prevents the formation of a concave meniscusin the fluid gathered within the features. FIG. 4G shows the wafer 301after it has been dried (step 124) and showing no signs of featurecollapse.

FIGS. 5A-5H show an example of a wafer 301 with high aspect rationanostructures 302 at selected steps of another embodiment of thedisclosed invention. FIG. 5A shows the result after step 102 has beenperformed. A layer of silicon based material 306 has been formed on thewafer 301. There are many processes that may be used to form the siliconbased layer 306. For example, the layer may be formed by physical vapordeposition, chemical vapor deposition, electrochemical deposition ormolecular beam epitaxy. While FIG. 5A shows a single uniform siliconbased layer 306, it is important to note that multiple layers ofmaterials may be used depending on many factors, such as the intendeduse of the circuit being fabricated or the specific fabrication processbeing used. Examples of multilayer structures include a stack consistingof TEOS at the top followed by SiN, then PolySi, and then Si at thebottom, or a stack consisting of SiN at the top followed by TEOS, thenanother layer of SiN, then PolySi, and then Si at the bottom.

FIG. 5B shows the wafer 301 with the layer of silicon based material 306and a photoresist layer 305 that has been patterned. A photoresist layer305 is deposited on the silicon based material, often using a spincoating process and the photoresist is patterned using photolithography(step 104). The photoresist layer 305 is used as a mask to determinewhat silicon based material to remove and what to leave behind duringthe etching process. A wet or dry etching process (step 108) may be usedto remove the material not covered by the photoresist 305.

FIG. 5C shows the wafer 301 after the etching, showing thenanostructures 302 formed by the etching (step 108). At this point thephotoresist 305 is still present. After the etching, the photoresistmaterial 305 may then be removed (step 112). The photoresist 305 may beremoved by a chemical stripping process or by an ashing process.

FIG. 5D shows a wafer 301 and four columns of the silicon based materialthat remain after etching and photoresist removal (step 112). Thecolumns of the silicon based material make up the nanostructures 302. Itis important to note that while the figures show a particular example offeatures that may be etched on the wafer 301, other numbers and types offeatures are possible.

FIG. 5E shows the wafer 301 undergoing a wet cleaning during wetprocessing to clean etch or ash residue from previous processes. The wetcleaning chemicals 501 may include: aqueous, semi-aqueous or organicsolutions of chemicals or combination of chemicals including HCl, HF,NH₄F, NH₃ aqueous solution, H₂SO₄, H₂O₂, for example.

FIG. 5F shows the surfaces of the nanostructures 302 being made morehydrophobic by depositing surface modifying agents 303 during wetprocessing (step 126) after the wet cleaning shown in FIG. 5E. Oneparticular example of a procedure to make the surfaces of thenanostructures 302 more hydrophobic is: 1) rinsing away the wet cleaningchemicals 501 with DI water, 2) replacing DI water with isopropylalcohol, 3) replacing isopropyl alcohol with HFEs, 4) immersing thewafer into 0.01-50% by weight of FOTS in HFE for approximately 2seconds-10 minutes, and 5) rinsing with HFE. An alternative exampleis: 1) rinsing away the wet cleaning chemicals 501 with DI water, 2)replacing DI water with organic solvents that don't contain an —OHgroup, but are also miscible with DI water (Examples of solvents havingsuch properties include: DMF, DMAC, acetone, NMP.), 3) immersing thewafer into 0.01-50% by weight of FOTS in the organic solvent solutionfor approximately 2 seconds-10 minutes, and 4) rinsing with the organicsolvent. Item 502 of FIG. 5F represents the chemicals used during theprocedure of making the surfaces of the nanostructures 302 morehydrophobic.

FIG. 5G shows the step to re-introduce DI water 304 into the hydrophobicnanostructures 302 after FIG. 5F to exploit the high Laplace pressuregenerated by convex water meniscus inside the hydrophobic nanostructures302 to prevent the nanostructures 302 from collapse during the dryingprocess. One particular example of doing so is: 1) replacing the HFEwith isopropyl alcohol, and 2) replacing isopropyl alcohol with DIwater. Since isopropyl alcohol has a lower surface tension than DI waterand is miscible with DI water, immersion in isopropyl alcohol first thenin DI water can introduce DI water into the hydrophobic nanostructures302 through diffusion process. Alternatively, if the step represented byFIG. 5F ends with an organic solvent that does not contain —OH group,but miscible with water, a simple DI water wash step can be appliedhere.

FIG. 5H shows intact high density high aspect ratio nanostructures 302after drying.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, modifications andvarious substitute equivalents, which fall within the scope of thisinvention. It should also be noted that there are many alternative waysof implementing the methods and apparatuses of the present invention. Itis therefore intended that the following appended claims be interpretedas including all such alterations, permutations, modifications, andvarious substitute equivalents as fall within the true spirit and scopeof the present invention.

What is claimed is:
 1. A method of processing a wafer used infabricating semiconductor devices, said method comprising: forming highaspect ratio features in a silicon based layer on the wafer, wherein thehigh aspect ratio features are formed utilizing a dry etching process;making sidewalls of the high aspect ratio features more hydrophobicwithout needing to wet the high aspect ratio features before the makingthe sidewalls of the high aspect ratio features more hydrophobic,wherein the sidewalls of the high aspect ratio features are made morehydrophobic prior to performing wet processing of the high aspect ratiofeatures of the wafer; performing wet processing of the high aspectratio features of the wafer; and subsequently drying the wafer.
 2. Themethod of claim 1, wherein the making the sidewalls of the high aspectratio features more hydrophobic is accomplished by depositing a primeron the wafer, wherein the primer makes the sidewalls more hydrophobic bychemically altering the surface of the high aspect ratio features. 3.The method of claim 2, wherein the primer is deposited on the wafer byway of vapor deposition.
 4. The method of claim 3, wherein thedepositing of the primer on the wafer prior to the wet processingcomprises: generating a primer solution by mixing a solution of 0.1-50%by weight of C₈F₁₃H₄SiCl₃ (FOTS) in anhydrous n-hexane; and co-heating aportion of the primer solution with the wafer such that the primersolution evaporates and the FOTS molecule reacts with the sidewalls ofthe-features.
 5. The method of claim 2, wherein the primer comprises aself-assembled monolayer.
 6. The method of claim 2, wherein the primercomprises a film, and wherein the film comprises at least one of:hexamethyldisiloxane, an alkoxysilane or an alkylsilane.
 7. The methodof claim 1, wherein the making the sidewalls more hydrophobic isaccomplished by roughening the sidewalls.
 8. The method of claim 7,wherein the roughening of the sidewalls is accomplished by way ofexposing the wafer to at least of: polymerizing plasma, hydrogenfluoride vapor or fluoride based plasma etch.
 9. The method of claim 7,wherein the roughening of the sidewalls is accomplished by reactive-ionetching.
 10. The method of claim 1, wherein forming of the high aspectratio features comprises: forming a photoresist patterned mask over thesilicon based layer; and etching the high aspect ratio features into thesilicon based layer.
 11. The method of claim 10, wherein the forming thehigh aspect ratio features further comprises stripping the photoresistpatterned mask.